Spontaneous Ignitions and Combustions

Spontaneous ignition is the bane of the fire investigator. If spontaneous ignition has occurred then the origin and cause of a fire may not be due to a conscious act of an individual or a mechanical failure but is the result of a rare set of circumstances that may not have left any evidence of their previous existence.

Spontaneous ignition can arise from several sets of conditions involving liquids, gases and solid materials.

Liquid wood finishing chemicals contain vegetable oils, which absorb oxygen from the atmosphere as they form a plastic protective film over the surface being covered. The "drying" reaction involves a large surface area and the heat released from oxygen absorption is spread out over the large area. If the oxygen absorption reaction occurs in a confined space with combustible materials then the propensity for a fire is created. In essence the heat releasing chemical reaction generates heat more quickly than the combustible material can loose it. Eventually the combustible material will reach its ignition temperature and the first flames develop. The process may be hours or days before ignition temperatures are reached. One of the more common sources of this problem are found in wood finishing chemicals which use unsaturated vegetable oils for forming the final protective film. Linseed, tung and peanut oils are three of the more common oils which can self heat under the correct set of circumstances.

There are certain combinations of liquid chemicals that are known as hypergolic mixtures. These combinations have been used in military weaponry and space exploration applications as power systems. The liquids only need to be pumped into a rocket motor nozzle and they self ignite on mixing.

Cryogenic fluids such as liquid oxygen can also cause self ignition if they are brought into contact with organic materials.

Any solid materials that are combustible or oxidizable can represent a fire hazard when they are very finely divided. Virtually all powdered metals if finely enough divided, can be considered pyrophoric. (spontaneously burst into flame upon contact with oxygen) Elementary chemistry teaches that the rate at which a chemical reaction proceeds is proportional to surface areas in contact. As a given weight of material is sub divided into smaller and smaller pieces the surface area per unit of weight increases. With very finely divided materials of very high surface areas, moisture and static electricity can provide the heats required to create ignition temperatures.

Numerous solids that are either combustible or oxidizable and are of small enough particle size to form an air borne dust, can create explosive gas mixtures with air. Many gases are combustible and when mixed with air in the proper proportions can form an explosive or flammable mixture. For dusts and gases there is usually a range of concentrations of the flammable entity between which combustion can occur. Outside of the range of flammability there is either not enough oxygen for combustion to occur or not enough fuel to sustain combustion.

Dust explosions can often occur in very dry environments favoring static electric accumulations that can set off an explosion and fire leaving little in the way of physical evidence of the event.

Gases generated by biological activities can collect and form mixtures with the air that are capable of combustion if ignited by either a pilot flame or the spark of an electric switch. Sewer gases such as methane and hydrogen sulphide are two of the commonly encountered biologically derived gases that must be safely vented from domestic sewage systems to avoid just such dangerous accumulations.

We have investigated, researched and occasionally verified experimentally, all three forms of fire origins in our years of practice.